By virtue of excellent magnetic properties and economy, rare earth permanent magnets find use in many areas of electric and electronic equipment. Recently the amount of these magnets produced has marked a dramatic increase. Among others, neodymium rare earth permanent magnets have lower feedstock costs than samarium-cobalt magnets because the primary element, neodymium exists in more plenty than samarium and the amount of cobalt used is smaller. They also have much better magnetic properties than samarium-cobalt magnets. For this reason, the neodymium rare earth permanent magnets are now applied not only to small-sized magnetic circuits where samarium-cobalt magnets have been used, but also to the fields where hard ferrite or electromagnets have been used. Also in the area of motors in compressors for use in air conditioners and refrigerators, a transition from traditional induction motors and synchronous rotating electric machines using ferrite magnets to DC brushless motors using neodymium rare earth magnets is taking place for the purposes of increasing energy efficiency and reducing power consumption.
However, R—Fe—B permanent magnets have the drawback that they are readily oxidized in humid air within a short time since they contain rare earth elements and iron as the main components. When these magnets are incorporated in magnetic circuits, oxidative corrosion raises such problems as decreased outputs of magnetic circuits and contamination of peripheral equipment with the rust resulting therefrom. Then, rare earth magnets are generally surface treated prior to use. Suitable surface treatments made on rare earth magnets include electroplating, electroless plating, and even Al ion plating and various coating processes. The environmental factor to which R—Fe—B permanent magnets are exposed during the process is mainly temperature or humidity.
In industrial motors and air conditioner compressor motors, on the other hand, there exist environmental factors inherent to the environment where rare earth permanent magnets are used. For example, rare earth permanent magnets are always exposed to chemical fluids such as cutting fluids or mixtures of refrigerant and refrigerating machine oil at high temperature and high pressure. Rare earth permanent magnets must be highly reliable, typically fully corrosion resistant in such unique environments.
Particularly when rare earth permanent magnets are used in linear motors for machine tools, it is believed that they offer high acceleration and high-speed rotation capabilities, enabling higher speed machining than in the prior art. It is often the case that on use, industrial motors are exposed not only to compression gases like fluorocarbons such as hydrofluorocarbons (HFC), but also to chemically active gases such as pure hydrogen and pure ammonia.
In the case of linear motors for use in high-speed machining, unless magnets have sufficient resistance to cutting fluids, the magnets may undergo progressive corrosion reaction with cutting fluids during long-term operation and degrade in magnetic properties, so that the motors fail to exert their performance to a full extent. Similarly, in the case of motors for use in an atmosphere having a certain partial pressure of pure hydrogen or pure ammonia, unless magnets have sufficient corrosion resistance, magnets undergo progressive corrosion reaction during long-term operation and degrade in magnetic properties, so that the motors fail to exert their performance to a full extent.
Then, in these applications, it is under consideration to implement various surface treatments as mentioned above. There is a strong need for a surface treatment capable of providing sufficient corrosion resistance in an environment exposed on actual use.
Such a surface treatment, if established, makes it possible to enhance the efficiency and reliability of various industrial motors, and is of great significance.
When R-T-B permanent magnets are used in high-efficiency motors, the magnets are generally exposed to an environment where air is moist, typically a hot humid environment. Magnets are also exposed to a special environment when high-efficiency motors are used in air conditioner compressors using both a HFC or HCFC refrigerant and a refrigerating machine oil such as mineral oil, ester oil or ether oil. A method for preparing a rare earth permanent magnet for use in such a special environment is disclosed in JP-A 2002-57052.
There is still a desire to have a rare earth permanent magnet providing cutting fluid resistance with respect to water-miscible metalworking agent compositions, especially amine-containing water-miscible cutting fluids.